Active-type range finding devices are widely used with recent compact cameras. A range finding device of this type has a light projector for projecting a spot of light or a line of light toward an object, and a light receiver for receiving light reflected from the object. The position of the reflected light incident upon the light receiver changes with object distance so that the object distance can be measured by detecting the incident position of the reflected light. As a light source of the light projector, an LED (light emitting diode) is mainly used which emits nearinfrared light from its p-n junction. As a light receiving element of the light receiver, a semi-conductor PSD (position sensitive detector) is often used which has a filter mounted on the light receiving surface thereof for transmission of near-infrared light. A PSD has two output terminals each outputting a current corresponding to the intensity and position of the incident light. By calculating the ratio between these two channel currents or voltages, a signal dependent only upon the incident position of near-infrared light can be obtained. The intensity of a spot of light reflected from an object changes with the object distance. Thus, the intensities of two signals outputted from a PSD may sometimes become too large or small, which reduces measurement precision. For this reason, a gain control amplifier is provided for each channel to obtain a signal having an adequate dynamic range. The gain of each gain control amplifier is automatically adjusted by feeding back a portion of each output signal of the PSD.
In a conventional range finding device using a PSD such as described in Japanese Patent Laid-Open Publs. Nos. 57-158508 and 57-182112, near-infrared light from an LED is applied to an object, and two channel currents outputted from a PSD are converted into corresponding voltages. The two channel voltages are amplified by corresponding gain control amplifiers and thereafter logarithmically compressed. Using the difference between the two logarithmically compressed channel signals, there is obtained the position of the light incident upon the PSD that corresponds to the object distance. There is disclosed in Japanese Patent Laid-Open Publ. No. 59-90012 a simple method of calculating the ratio between two channel analog signals outputted from two gain control amplifiers by using the charge-discharge time of a capacitor. There is also proposed a method of calculating the ratio between two signals and A/D converting by using one of the two signals as a reference for A/D conversion. There is also known a method of improving the precision of range finding by repeating a plurality of measurements and determining the position of incident light representative of object distance by using an average value of the plurality of obtained measurement data.
The above-described techniques have the following disadvantages. In the technique using logarithmic compression, it becomes necessary to use a logarithmic amplifier, differential amplifier, logarithmic expansion amplifier and the like for each channel, resulting in a large circuit. In addition, the S/N ratio of a logarithmic amplifier is worse than that of a linear amplifier so that it is difficult to improve the precision of range finding. For the technique using the charge/discharge time of a capacitor, although the circuit can be made smaller than when using a logarithmic amplifier, there are required relatively complicated division circuits for obtaining the ratio between the two channel signals.
A PSD receives not only near-infrared light reflected from an object but also unnecessary ambient light from the photographic scene. There is known a method of avoiding the influence of unnecessary ambient light, wherein a pulsed spot of light having a predetermined frequency is generated by an LED for example, and two channel analog signals are passed through high-pass filters to derive only the signal components having the same frequency. With this method, however, it is difficult to remove noise components completely. In addition, for an object at a great distance, each channel signal contains large unnecessary light components, thereby lowering the precision of measurement. This measurement precision is also lowered by the influence of components present in signal processing circuits such as gain control amplifiers.
Each channel signal for an object having a high reflectivity and which is located at a near distance becomes too large so that even if the gain of the gain control amplifier is set at a minimum value, the channel signal cannot be adjusted within an adequate output dynamic range. For such an object, even if near-infrared light is projected after a gain adjustment toward an object for range finding, measurement with good precision is not possible. Even after the gain of a gain control amplifier has been adjusted properly for an object, if the object should move, the near-infrared light beam may reach a different object having a different reflectivity. Also in such a case, it often occurs that the output signal from the gain control amplifier becomes too large so that the proper output dynamic range cannot be obtained. To solve this problem, the gain of the gain control amplifier should be adjusted again and range finding carried out thereafter. In this case, however, if the reflection factor changes during this period, only the gain adjustment must be repeated resulting in the range finding being disabled. Particularly in the case wherein a plurality of range finding operations are repeated after the gain is adjusted, the average value of the plurality of obtained measurement data is used in determining the incident light position in order to improve the range finding precision, there is a good possibility that each channel signal will become excessive during such measurements, again resulting in range finding being disabled. There is known a camera which automatically moves the taking lens to a pan focus position in such an unsuitable state of range finding. But with a camera of this type, an out-of focus exposure is likely to be made of an object at a near distance.
An LED for emitting light of long wavelength, such as a near-infrared light, has a negative temperature-light output characteristic. Thus, the amount of emitted light at the start of the period of driving the LED is less than that in the stable state. It is therefore impossible to set an optimum gain for a range finding device of the type in which an LED is driven a number of times and the gain of a gain control amplifier is determined by using light emission from the outset and thereafter a distance measurement is made. In this case, because of improper gain, there will be the difficulty that the output signal from the gain control amplifier for each channel does not reach an expected level at the outset of range finding. It is possible to judge object distance to be infinite if the output signal from a PSD is null or extremely small. However, if such a judgment is made when the amount of LED light emission is small, then an object which is actually within a detectable range if to be at infinity, which of course is an erroneous measurement.
Many recent compact cameras have microcomputers built therein. With such cameras, a microcomputer executes a photographing sequence to control range finding, photometry, exposure and film transport, upon actuation of a shutter button. During this sequence, the microcomputer collects data from various circuit portions of a range finding device, photometry device, film transport device and the like. During the photographing sequence, the microcomputer can reliably retrieve data from various circuit portions of the photometry device, film transport device and the like, except the measured data from the range finding device, because such data has a wide dynamic range. In consideration of this circumstance, conventionally an active-type range finding device has been provided with a function to adjust the intensity of nearinfrared light to be emitted by an LED to a proper value, a function to amplify the output signal from a PSD to a proper level, a function to measure a plurality of points in a photographic scene and select a distance datum at an optimum point with priority over other points, and other functions. Therefore, it is inevitable that the circuit becomes large in size and the manufacturing cost becomes high.